1. Field of the Invention
The present invention relates to polarization independent optical isolators for shielding return beams which are reflected by optical systems (i.e. optical fiber communication systems utilizing semiconductor lasers or optical fiber amplifiers).
2. Description of the Related Art
Semiconductor lasers, which are used as light sources for performing input and output operations in optical fiber communication systems and optical disks, suffer from unstable oscillations when return beams are reflected by the optical system. These return beams are caused by factors such as an end surface of an optical fiber, connecting points between optical fibers, coupling lenses, and optical connectors. The unstable oscillation caused by return beams leads to significant impairment of performance such as an increase in noise and fluctuations in the output.
Optical isolators of various types have been developed so as to remove such instability in the operation of the semiconductor lasers. One such type of optical isolator is the polarization independent optical isolator. The polarization independent optical isolator may utilize the separation/synthesis of the ordinary ray (hereinafter "O ray") and the extraordinary ray (hereinafter "E ray") through the use of plate-like birefringent crystals such as rutiles and calcites. An advantage of the use of this type of polarization independent optical isolator is that it exhibits isolation effects upon all planes of polarization without being dependent on the direction of polarization.
For example, Examined Japanese Patent Publication No. Sho. 60-49297 discloses an optical isolator in which a first plate-like birefringent crystal, a magneto-optical material (i.e. Faraday rotator), an optically active crystal, and a second plate-like birefringent crystal are arranged in order from an incident end. That optical isolator has a permanent magnet for magnetizing the magneto-optical material. Unexamined Japanese Patent Publication No. 2-46419 and Unexamined Japanese Patent Publication No. 2-68515 each disclose an optical isolator in which not only two or more magneto-optical materials and three or more plate-like birefringent crystals are arranged, but also a permanent magnet for magnetizing the magneto-optical materials is provided.
In such conventional optical isolators, the direction and angle of rotation of a plane of polarization by the magneto-optical material as well as the direction and amount of displacement of polarization by the plate-like birefringent crystal have not been adequately studied. Accordingly, it has been found that the plane of polarization of an incident light beam can change as it passes through an optical isolator. It has also been found that differences in the path length (i.e. dispersion of a polarized wave) can also be caused by passing through an optical isolator. The particular effects on the beam depend on the polarization of the incident light beam. The result of the change in the polarization and the dispersion of the polarized wave is that the signal beam can become disturbed.
Moreover, conventional optical isolators fail to take into consideration methods of obviating the fluctuations in performance with respect to different operating temperatures and changes in the wavelength of the incident light beam. Therefore, there are no prior art optical isolators which are highly reliable and that can be used satisfactorily in many applications.